This invention relates to an electromagnetic fuel injector for use in an electronically controlled fuel injection system of a single- or multiple-point type for an internal combustion engine in an automotive vehicle.
FIG. 1 shows a vertical sectional view of a conventional electromagnetic fuel injector designated by reference number 1. The electromagnetic fuel injector 1 is provided with a fuel injection nozzle 3 at its front end. A valve housing 2 is provided with a fuel passage 4 extending along its axis, and a plunger-like valve body 5 is inserted into the fuel passage 4. An armature 6 is fixed to the rear end of the valve body 5. The valve housing 2 is retained by an electromagnetic housing 7 at its front position. A fixed magnet core 8 and an exciting coil or winding 9 are accommodated in the electromagnetic housing 7 at its rear portion. In response to the control signal inputted from a terminal 10 to the exciting coil 9, the valve body 5 is effective to axially reciprocate for discharging pressurized liquid fuel from the fuel injection nozzle 3. The inner surface of the nozzle 3 serves as a valve seat 3a which is adapted to come into contact with a valve member 5a of the valve body 5. The cylindrical inner surface of the fuel passage 4 serves to guide a slide portion of the valve body 5. The front portion of the valve housing 2 is protected by a cover 7a and the rear portion thereof is fixed to the front portion of the electromagnetic housing 7 with an O-ring seal 11 and a non-magnetic spacer 12 interposed. The outer circumference of the valve body 5 is formed with a flange 5b on the front side of the spacer 12, and the flange 5b is adapted to come in to contact with the front surface of the spacer 12 when the valve body 5 moves up to the rearmost position. The electromagnetic housing 7 as a yoke is formed of a ferromagnetic material, and the exciting coil 9 is housed in a space between the electromagnetic housing 7 and the fixed magnet core 8 with O-ring seals 13 and 14 interposed. The fixed magnet core 8 is also formed of a ferromagnetic material and is provided with an axial through-hole as a fuel passage 15. A compression spring 16 is inserted into the front portion of the axial through-hole so as to normally bias against the rear end of the armature 6 and hold the valve body 5 in a closed position. The compression spring 16 abuts against the front end of a sleeve 17 which is carried in the axial through-hole of the fixed magnet core 8. A fuel filter 18 is provided at the rear end of the fuel passage 15.
The stroke S of the valve body 5 is determined in such a manner that the valve body 5 abuts against the valve seat 3a and both positions of the rear end of the valve housing 2 and the rear end of the valve flange 5b are so suitably adjusted as for the distance between both of the rear ends to become S. An air gap D is defined between the rear end of the armature 6 and the front end of the fixed magnet core 8 so as for the valve body 5 not to be influenced by the residual magnetism of the fixed magnet core 8 when the valve body 5 moves forwardly from its opening position. In order to suitably select the spacer 12, the combination size A of the electromagnetic housing 7 and the fixed magnet core 8, and the combination size B of the valve housing 2, the flange 5b and the armature 6 are respectively measured, and the thickness C=(B+D)-A of the spacer 12 is calculated. The size A is the combination of two elements, and it is hard to accurately measure the axial dimension of the central bore. The size B is the combination of three elements, and it is also hard to accurately measure the axial dimension of the central bore since the valve housing 2 and the valve body 5 are not fixed. In the case that the thickness of the spacer 12 is selected after measurement of the sizes A and B, the problem seems to be that many kinds of spacers 12 must be prepared per one micro meter so as to increase the accuracy of the size D.